1. Field of the Invention
The present invention relates to an optical communication system, and particularly to an optical communication system with an optical output level control function for controlling the optical output level of an access section in the optical transmission, and to a two-way optical communication system for carrying out 1×L two-way communications between one optical transmitting/receiving unit (master station) and L optical transmitting/receiving units (slave stations). Here, an optical signal transmitted from the master station to a slave station is called a downlink optical signal, and an optical signal transmitted from a slave station to the master station is called an uplink optical signal.
2. Description of the Related Art
FIGS. 1 to 5 each show an example of a conventional optical communication system with an optical transmitter. FIG. 1 shows a configuration of an optical communication system including an optical transmitter with an optical splitter for splitting an optical signal to optical signals with equal optical output levels, and optical receivers connected to the optical transmitter via optical fibers.
FIG. 2 shows a configuration of an optical communication system including an optical transmitter with an optical splitter for optically splitting an optical signal to signals with equal optical output levels, optical couplers connected to the optical transmitter via optical fibers, and optical receivers connected to the optical couplers via optical fibers. FIG. 3 shows a calculation example of a minimum optical reception level in FIG. 1, and FIG. 4 shows a calculation example of a minimum optical reception level in FIG. 2.
In the present specification, although optical signal is split by an optical transmitter, by a remote variable splitter or by an optical coupler, optical signal passing through the splitting do not always have the same optical levels at the optical receivers. In the present specification, the minimum level among the levels received by the optical receivers is called a minimum optical reception level.
The optical communication system of FIG. 1 includes an optical transmitter 201 with an optical splitter 2012 for splitting an optical signal fed from an optical fiber 2011 to N optical signals with the same optical output level, and N optical receivers 401, 402, . . . , and 40N connected to the optical transmitter 201 via N optical fibers 301, 302, . . . , and 30N.
FIG. 3 shows a calculation example of obtaining the minimum optical reception level Pin_min (dBm) in the optical communication system as shown in FIG. 1. Here, the optical receiver connected to the optical transmitter 201 via an ith (i=1 to N) optical fiber is denoted as an optical receiver (i). Assume that the optical output level of the optical fiber 2011 is Pout (dBm), the transmission loss of the ith optical fiber is L(i) (dB), and the optical reception level of the optical receiver (i) is Pin(i) (dBm). All the coupling ratios of the optical output levels of the optical splitter 2012 are 1/N. This is because the optical splitter 2012 splits the input optical signal to the same output levels, and the number of the optical receivers connected to the optical splitter 2012, that is, the splitting number of the optical splitter 2012 is N.
The coupling ratio gives the optical loss of the optical splitter 2012 of 10 log10N (dB) per branch. Thus, the relationship between the optical output level Pout (dBm) and the optical reception level Pin(i) (dBm) at the optical receiver (i) is given by the following equation.Pout−10 log10N−L(i)=Pin(i)(i=1 to N)  (1)
Since the left-hand terms Pout and 10 log10N of equation (1) are constant for all the optical receivers, the optical reception level becomes minimum when L(i) is maximum. By denoting the minimum optical reception level as Pin_min (dBm), and the transmission loss when the optical reception level of the optical receiver is minimum as Lmax (dB), the following equation holds.Pout−10log10N−Lmax=Pin_min  (2)
Next, the configuration of FIG. 2 will be described. The optical communication system of FIG. 2 comprises: an optical transmitter 501 including an optical splitter 5012 for splitting the optical signal fed through an optical fiber 5011 to optical signals with the same optical output level; N optical couplers 701, 702, . . . , and 70N connected to the optical transmitter 501 via N optical fibers 601, 602, . . . , and 60N; m1 optical receivers 911, 912, . . . , and 91m1 connected to the first optical coupler 701 via m1 optical fibers 811, 812, . . . , and 81m1; m2 optical receivers 921, 922, . . . , and 92m2 connected to the second optical coupler 702 via m2 optical fibers 821, 822, . . . , and 82m2; and mN optical receivers 9N1, 9N2, . . . , and 9NmN connected to Nth optical coupler 70N via mN optical fibers 8N1, 8N2, . . . , and 8NmN.
FIG. 4 shows a calculation example of obtaining the minimum optical reception level Pin_min (dBm) in the optical communication system as shown in FIG. 2. Here, the optical receiver connected to the optical transmitter 501 via an ith optical fiber and jth optical fiber (j=1 to mi) is denoted as an optical receiver (i,j) (i=1 to N, j=1 to mi).
Assume that the optical output level of the optical fiber 5011 is Pout (dBm), the total transmission loss of the ith and jth optical fibers is L(i,j) (dB), and the optical reception level of the optical receiver (i,j) is Pin(i,j) (dBm). All the coupling ratios of the optical output levels of the optical splitter 5012 are 1/N.
This is because the optical splitter 5012 splits the input optical signal to the same output levels, and the number of the optical couplers connected to the optical splitter 5012, that is, the splitting number of the optical splitter 5012 is N. Thus, the coupling ratio gives the optical loss of the optical splitter 5012 of 10 log10N (dB) per branch. In addition, since the optical couplers 701, 702, . . . , and 70N each have the splitting number mi, and the optical couplers each split the optical input into the same output levels, the optical loss of the optical couplers 701, 702, . . . , and 70N is given by 10 log10mi (dB) per branch.
Accordingly, the relationship between the optical output level Pout (dBm) and the optical reception level Pin(i,j) (dBm) at the optical receiver (i,j) is given by the following equation.Pout−10log10N−{10log10mi+L(i,j)}=Pin(i,j) (i=1 to N,j=1 to mi)  (3)
Since the left-hand terms Pout and 10 log10N of equation (3) are constant for all the optical receivers, the optical reception level becomes minimum when 10 log10mi+L(i,j) is maximum. By denoting (i,j) when the optical reception level of the optical receiver is minimum as (u, v), the following equation holds.Pout−10log10N−{10log10mu+L(u,v)}=Pin_min  (4)
As described above, the optical reception levels of the optical receivers of the conventional optical communication system depend on the transmission loss L(i), splitting number and transmission loss {10 log10mi+L(i,j)}.
The optical communication systems as shown in FIGS. 1 and 2, however, have a problem of high cost. This is because in the optical communication systems as shown in FIGS. 1 and 2, it is necessary for the optical receivers with smaller optical reception levels to be more sensitive than the optical receivers with greater optical reception levels to receive the lower intensity light.
Thus, the optical communication systems as shown in FIGS. 1 and 2 have the problem of increasing the cost because of the highly sensitive optical receivers.
Furthermore, the optical communication systems as shown in FIGS. 1 and 2 have another problem of having low output efficiency of the optical signals as the optical communication systems in their entirety. This is because receiving the light with the intensity more than necessary, the optical receivers with higher optical reception levels in the optical communication systems as shown in FIGS. 1 and 2 waste the light as the total optical communication systems.
FIG. 5A shows a conventional two-way optical communication system including one optical transmitting/receiving unit 1 and L optical transmitting/receiving units 2-1 to 2-L, which are connected in a 1×L fashion via an optical fiber 3-0, 1×L optical coupler 4, and L optical fibers 3-1 to 3-L. The optical coupler 4 splits a downlink optical signal with a wavelength λd transmitted from the optical transmitting/receiving unit 1 to L parts, and transmits them to the optical transmitting/receiving units 2-1 to 2-L. Reversely, the optical coupler 4 combines uplink optical signals with a wavelength λu transmitted from the optical transmitting/receiving units 2-1 to 2-L, and transmits the combined signal to the optical transmitting/receiving unit 1.
The optical coupler 4 has equal 1/L coupling ratios so that the downlink optical signal through the optical fiber 3-0 is split to the optical fibers 3-1 to 3-L at the same level, and the uplink optical signals via the optical fibers 3-1 to 3-L are combined to the optical fiber 3-0 at the level with the same coupling loss subtracted.
However, the optical fibers 3-1 to 3-L connecting the optical coupler 4 with L optical transmitting/receiving units 2-1 to 2-L as shown in FIG. 5A have different transmission losses in accordance with the transmission distances, though the optical coupler 4 has the equal coupling ratios. Accordingly, although the downlink optical signals have the same level at the input terminals of the optical fibers 3-1 to 3-L as illustrated in FIG. 5B, the optical transmitting/receiving units 2-1 to 2-L have different optical reception levels because of the variations in the transmission loss. On the other hand, as for the uplink optical signals, even if they have variations in the optical levels at the output terminals of the optical fibers 3-1 to 3-L, they are combined as they are and received by the optical transmitting/receiving unit 1. Specifically, as illustrated in FIG. 5C, the optical reception levels of the signals sent from the optical transmitting/receiving units 2-1 to 2-L differ at the optical transmitting/receiving unit 1.
Therefore the optical transmitting/receiving unit 1 and optical transmitting/receiving units 2-1 to 2-L each require a highly sensitive, wide dynamic range photo-detection circuit that can cope with the variations in the optical reception levels and the minimum optical reception level, thereby increasing the system cost.
As a technique to circumvent such problems, Japanese Patent Application Laid-open No. 4-269023 (1992) discloses a method in which slave stations transmit constant level signals to enable a master station to adjust optical attenuation and to transmit different level signals to the slave stations. In addition, Japanese Patent Application Laid-open No. 11-136192 (1999) discloses a method in which a master station adjusts the transmission level for each slave station so that the reception levels of the individual slave stations with different transmission losses become equal.
However, configuring a 1×L optical coupler 4 by combining Mach-Zehnder interferometer type 1×2 optical couplers, for example, presents a problem of the wavelength dependence of the individual 1×2 optical couplers. Specifically, the equal coupling ratio for the downlink optical signals with the wavelength λd is not always equal for the uplink optical signals with the wavelength λu. Furthermore, when the downlink optical signals and the uplink optical signals are wavelength division multiplexed optical signals including multiple wavelengths, the coupling ratios are never equal for the wavelength division multiplexed optical signals because the coupling ratios vary in accordance with the individual wavelengths.